Many kinds of data buses have been developed for use in computer systems. Most of the early examples required that the computer system be powered down when attaching or detaching a device from the data bus.
More recently, buses have been designed that allow devices to be attached not only when the computer system is powered on, but moreover when the computer system is running. Software may recognize newly attached devices and enable their use without having to reboot the operating system. Buses that operate in this manner may be referred to as hot-plug-in data buses. Common examples of hot-plug-in data buses include the Universal Serial Bus (USB) (Universal Serial Bus Specification Compaq/Intel/Microsoft/NEC Revision 1.1, published Sep. 23, 1998), the Personal Computer Memory Card International Association (PCMCIA) PC Card bus (PCMCIA PC Card Standard Specification Release 2.01, published November 1992), and the Institute of Electrical and Electronic Engineers IEEE-1394 bus (IEEE Standard for a High Performance Serial Bus, IEEE Std. 1394-1995, published Aug. 30, 1996).
Once a device is attached to a hot-plug-in data bus, conventional operating system software schedules the periodic examination of the device (called “polling”) to see if the device wishes to transfer data. However, there are times when it would be advantageous for the computer system to enter into a reduced power state. The computer system operating system may be prevented from allowing the entry into a reduced power state due to the polling schedule. The user would be required to physically detach the device or devices from the computer system in order to permit the entry into the reduced power state. Alternatively, non-standard operating system software could be written. Neither of these two alternatives are desirable.